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Synoptic weather patterns and their relationship to high ozone concentrations in the Taichung Basin
Atmospheric Environment 35 (2001) 4971–4994
Synoptic weather patterns and their relationship to high ozone concentrations in the Taichung Basin Wan-Li Cheng Department of Environmental Science, Tunghai University, Taichung 407, Taiwan, ROC Received 1 September 2000; received in revised form 10 May 2001; accepted 26 May 2001
Abstract Frequent high ozone days (defined as daily maximum ozone concentration X80 ppb) during recent years in the Taichung Basin have caused much concern. High ozone days occur mainly during autumn and spring. Statistically, there is no clear linear relationship between a single meteorological variable and ozone concentration. In this study, data from 1996–2000 has shown that high ozone concentrations occur during two types of synoptic weather patterns. The first type is a continental cyclone emanating from mainland China, the southern part of it swept towards Taiwan by easterly winds. The second pattern is a tropical depression moving northwards toward the region, the northern part of it affecting Taiwan via easterly winds. Both types cover Taiwan with easterly winds, which are blocked by the Central Mountain Ranges (altitude of 2000–3000 m). The ranges create lee cyclogenesis to the west, which is unfavorable for pollutant dispersion and leads to serious air pollution episodes. The statistical results of the synoptic weather patterns in relation to ozone concentrations are based on the 5 yr data (1996–2000). This was obtained from a network of air-pollution monitoring sites in the study area, while the vertical data come from two 3-day tethersonde experimental campaigns conducted during March and October 2000, measuring air pressure, air temperature, relative humidity, wind speed and direction, non-methane hydrocarbons, NOx and O3. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Synoptic weather patterns; Ozone concentration; Tethersonde observation; Meteorological conditions
1. Introduction Taiwan is one of the newly industrialized economies. It has a population of 23 million in an area of 36,000 km2, with the majority of people living in the main metropolitan areas such as Taipei City (located in the Taipei Basin), Kaohsiung City (an industrial port city) and Taichung City (located in the Taichung Basin). This paper analyses the Taichung Basin, a region of 30 km 40 km, as part of the Central Taiwan AirQuality Management Program (CTAMP). West-central Taiwan has a number of low ranges (the main ones being the Tadu and Baqua Hills, both approximately 300 m elevation) running roughly north–south near the west coast. To their east is the magnificent Central Mountain Ranges (2000–3000 m). E-mail address: [email protected] (W.-L. Cheng).
The low and high ranges enclose the Taichung Basin, in which lie Taichung City and the surrounding urban towns (population around 3.5 million) (Fig. 1). Industrial and traffic emissions in this region have severely degraded the air quality causing high levels of pollutants and a reduction in visibility in recent years (EPA Taiwan, 2000; Tsai and Cheng, 1999). It is estimated that about 94,000 t yr1 of nitrogen oxides (NOx) are released into the air from industrial and traffic emissions. In addition, two thermal power plants nearby are the main sources of NOx pollutants influencing the Taichung Basin although the plants are outside the basin itself (29,000 t yr1 for the Taichung power plant and 15,000 t yr1 for the Tonhsiao plant). It is estimated that about 167,000 t of non-methane hydrocarbons (NMHC) were released in 1997 by industrial and traffic emissions. The study here extends previous work on the effects of winds on sulfur concentration (Cheng, 2001) and their
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 2 9 5 - 3
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Fig. 1. Location of monitoring stations over Taichung Basin (Tonhsiao power plant located 25 km north of Fengyuan).
effect on ozone. The sources of NOx and NMHC are mainly located in the central and northern parts of the basin, as seen in Figs. 2 and 3. Ozone (O3) is a secondary chemical pollutant, formed from NOx and volatile organic compounds (VOCs) (especially NMHC) in the presence of strong solar radiation. A general reaction mechanism for ozone at the boundary layer involves reacting NOx and NMHC with solar radiation: NOx þNMHCþhnþMðN2 ; O2 Þ-O3 þother photochemicals;
ð1Þ
where NOx comprises NO and NO2, NMHC is nonmethane hydrocarbon, hn denotes a photon, and the M returns the molecule to its original state. Most studies show that its formation is correlated with solar radiation, air temperature, relative humidity, wind speed and direction (Seinfeld and Pandis, 1998; Cheng and Bai, 1998; Poissant et al., 1996; Lalas et al., 1987). The ozone and its precursors can be transported downwind toward an area unfavorable for pollution
dispersion, thus creating the potential for a high ozone concentration in an area that has little local NOx and NMHC. Thus, the impact of ozone pollution is not limited to the location of the emission source (Gu. sten et al., 1988; Kambezidis et al., 1998; Hastie et al., 1999). A comparison of the meteorological variables on high ozone days during 1996–2000 is shown in Fig. 4. As in the Liu et al. (1994) study, there is no clear linear relationship between the meteorological components of cloud amount, solar radiation, relative humidity, sunshine duration, wind speed or maximum temperature with ozone concentration. Computer models, such as the Urban Airshed Model (UAM) and the Industrial Source Complex Dispersion Models (ISC), have been applied to the problem of ozone distribution in the west-central Taiwan region in recent years, but without much success (Chang and Wang, 1998). The Photochemical Grid model has achieved some results but is hindered by lack of sufficient local information (Chang, 2001). Patel and Kumar (1998) indicated that these models are deficient, particularly in handling airflow across complex terrain. Hence, this study investigates the relationship between
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Fig. 2. Estimated NOx emissions in west-central Taiwan in 1997.
Fig. 3. Estimated NMHC emissions in west-central Taiwan in 1997.
synoptic weather patterns and high ozone days in the Taichung Basin to identify those patterns correlated with higher ozone levels. These results could potentially be useful to EPA Taiwan in formulating pollution abatement strategies for sites specifically affected by certain synoptic weather patterns, such as the Taichung Basin.
ozone formation. In spring (March, April, May), the cold anticyclones decline, the wind speed decreases and the temperature rises. High ozone days occur during this season. Synoptic patterns in spring are an extension of the winter ones, but due to variable weather, the extent of ozone formation will fluctuate. In summer (June, July, August), Taiwan is dominated by the Pacific Anticyclone. Afternoon showers are frequent. Although intense solar radiation and low wind velocity are favorable to ozone formation, the showers will suppress high ozone levels. In autumn (September, October, November), the Pacific Anticyclone begins to retreat. The Siberian cold anticyclone begins to develop while the southern tropical depression moves occasionally northwards, exerting the most influence over Taiwan. Strong solar radiation with low humidity during autumn makes it the most favorable season for ozone formation. Several classifications of synoptic weather patterns in relation to air quality have been developed in recent years (Lee and Yu, 2000; Liu et al., 1994; Comrie, 1992, 1990). In this study, 14 synoptic weather patterns affecting the Taichung Basin have been categorized after a thorough analysis of daily surface synoptic charts issued by the Central Weather Bureau Taiwan during 1996–2000 (Cheng and Bai, 1998). Of these patterns, Types III and VII are the two patterns that produce the highest ozone concentrations
2. Synoptic patterns and their relationship to high ozone concentrations The geographical location of the study region is on the border between Eurasian continent and the Pacific Ocean, i.e. on the boundary between the Earth’s largest land and water masses. Climatically, it straddles the temperate and tropical zones. As such, it is affected in winter months by the middle latitude weather patterns and the Westerlies. In summer months, it is influenced by the tropical depressions and the Easterlies. During the winter months (December, January, February), Taiwan is predominantly influenced by anticyclones from Siberia. Accompanying the anticyclones are other synoptic patterns, such as passing cold fronts and warm areas in advance of a cold front. There is high wind speed, low temperature and the sky is cloudy with occasional rain, which is unfavorable for
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Fig. 4. Meteorological variables versus peak O3 (ppb) for Taichung Basin during 1996–2000.
in the Taichung Basin. Type III, where the anticyclone is located northeast of Taiwan, has weak to moderate easterly winds covering the region. Type VII, where the northern extent of the tropical depression reaches the vicinity of Taiwan, also has weak to moderate easterly winds.
During easterly winds, such as those caused by Types III and VII, the winds are blocked by the Central Mountain Ranges and split into two currents which pass around the ranges to the north and south, while the depleting air to the west creates the characteristic lee trough (Lin, 2001). This could also modify
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micro-synoptic features significantly through such processes as lee cyclogenesis, which is unfavorable for pollutant dispersion, resulting in high ozone concentrations (Lee and Yu, 2000). Table 1 illustrates the frequency of high ozone occurrences. Among the 14 patterns, Type III has the highest frequency at 71.7% (208 out of 290 days), followed by Type VII at 66.7% (138 out of 207 days). It is also found that among Type III, high ozone pollution days (HODs) mainly occurred in autumn with the highest frequency (86.8%), followed by spring (71.1%), as shown in Table 2. A HOD is defined here as a day with a maximum ozone concentration greater than 80 ppb. The remaining days are non-HODs (NHODs). The concentration level of 80 ppb is the pollution critical-threshold index widely used by many countries such as Brazil, Canada, China, Japan, Poland, etc., but not the USA, which uses a level of 120 ppb. Statistical analysis of synoptic pattern Type III in the period 1996–2000 shows that there are significant differences between HODs and NHODs in meteorological variables (Table 3). HODs occur under light wind speeds, averaging 2.0 m s1 (23.0% less than NHODs), and show a large standard deviation in wind direction of 23.5 (30.6% more than NHODs). The maximum temperature during HODs was 28.81C (7.9% higher than NHODs), and the average cloud cover was low 38.2% (18.2% less than NHODs).
3. Tethersonde experimental campaigns In order to monitor high ozone concentration patterns for Type III as mentioned above, two experi-
Table 1 The frequency of occurrence of high ozone days with different synoptic weather patterns in the period 1996–2000
Type II Type III Type VII
Days
HODs
Frequency (%)
249 290 207
92 208 138
36.9 71.7 66.7
mental campaigns were conducted for three consecutive days: 30 March–1 April 2000 with a typical spring Type III pattern, and 9–11 October 2000 with a typical autumn Type III pattern. The meteorological conditions over the study area for these 3-day periods were analyzed, and the vertical profiles of nitrogen oxide and ozone were examined, including their diurnal variation. The tethersonde has a balloon-borne radiosonde facility to observe air pressure, air temperature, relative humidity, wind speed and direction. The radiosonde was supported by a 3 m3 hydrogen balloon, tethered on a Kevlar line and controlled by an electric capstan. On the ground were additional air pollutant (O3, NOx, NMHC) analyzers. The sampling altitudes were at ground level, 50, 100, 250, 400, and 600 m for the coastal sites at Longjin and Tachia, and ground level, 100, 300, 500, and 1200 m for the basin center site at Tsaotun. Samples were taken every 3 h, with one and a half hours required to complete the observation and analysis for one round. An operational diagram of the experimental equipment is shown in Fig. 5. The ozone analyzer used was the Advanced Pollution Instrumentation, Inc. Model API 400. The instrument is designed with a built-in daily calibration system with a two-point (0, 400ppb) calibration, and a multipoint (0, 100, 200, 300, 400 ppb) calibration prior to use, according to the manufacturer’s calibration procedure. The accuracy of this instrument is 71 ppb and the time required for the analysis of one sample was approximately 5 min. The nitrogen oxides analyzer used was the Advanced Pollution Instrumentation, Inc. API Model 200A. The Model 200A utilizes one small diameter photomultiplier tube and one reaction chamber which are time-multiplexed for NO and NOx measurements. The difference between the NOx and NO allows the calculation of NO2. The instrument was also calibrated prior to use, according to the manufacturer’s procedure. Its accuracy is 70.5% of the reading and the required time for the analysis was also about 5 min. The methane/non-methane hydrocarbon analyzer is made by the Dasibi Environmental Corporation, Model Table 3 Meteorological variables during HODs and NHODs in synoptic patterns Type III in the period 1996–2000
Table 2 The frequency of occurrence of high ozone days with synoptic pattern Type III in the period 1996–2000
Winter Spring Summer Autumn
Days
HODs
Frequency (%)
85 81 F 91
48 114 F 79
56.5 71.1 F 86.8
Max temp (1C) Cloud cover (%) Solar rad. (MJ m2) Sun (h) RH (%) 10–15 WD (s.d.) 10–15 WS (m s1)
HODs
NHODs
28.8 38.2 12.1 7.1 73.2 23.5 2.0
26.7 46.7 11.6 6.9 75.2 18.0 2.6
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W.-L. Cheng / Atmospheric Environment 35 (2001) 4971–4994 Table 4 Meteorological variables during HODs and NHODs in synoptic patterns Type III in the month of March 2000
Max temp (1C) Cloud cover (%) Solar rad. (MJ m2) Sun (h) RH (%) 10–15 WD (s.d.) 10–15 WS (m s1)
Fig. 5. Operational diagram of instruments.
302. The analyzer was calibrated prior to use, checked after each start up and also periodically as the instrument follows an automatic monitoring schedule. Its accuracy is 1.0% of full scale and the required time for the analysis was also about 5 min. All data used in this paper have undergone standard US EPA procedures of quality assurance and control to minimize errors, such as sensitivity analysis, computerized checks, reality checks, sample calculation, statistical checks, independent audits, etc. The instrumentation and the methodologies used to monitor the CTAMP tethersonde data sets have been described elsewhere (Cheng, 2000; Tsuang et al., 2000). 3.1. Experimental campaign I Statistical analysis of the synoptic pattern Type III in March 2000 also shows significant differences in meteorological variables between HODs and NHODs (Table 4). Although the wind speed was identical (2.1 m s1), HODs had a larger standard deviation in wind direction of 28.0 (56.4% more than NHODs). HODs’ maximum temperatures were 28.11C (15.2% higher than NHODs), and had a lower average cloud cover of 51.5% (20.2% less than NHODs). During 30 and 31 March 2000, a continental anticyclone moving eastwards over the East China Sea had its center located northeast of Taiwan, with weak
HODs
NHODs
28.1 51.5 12.3 7.3 61.5 28.0 2.1
24.4 64.5 10.4 5.5 71.5 17.9 2.1
easterly winds covering the area. This was a typical spring Type III pattern, with relatively weak pressure center (Fig. 6). 30 March 2000 was a clear day with solar radiation reaching 13.2 MJ m2. The temperature at Taichung City ranged from 18.11C to 27.81C. This gave maximum sea–land temperature differences of 5.41C (the sea temperature showed a 4.91C range over the day from a minimum of 17.01C at 06:00 h). All times hereafter are referenced to Taiwan Central Standard Time, which is 8 h ahead of UT. The second day was partly cloudy, with solar radiation slightly reduced to 11.3 MJ m2, and the maximum sea–land temperature difference was 4.71C at 15:00 h. In these two days, clear sea breeze circulations developed as shown from the wind field in the region (Fig. 7) and at the vertical profile (Fig. 8). On the afternoon of 30 and 31 March, the sea breeze inflow reached altitudes of 200–400 m. At ground level, the O3 concentration became highest at the southern (downwind) end of the basin at 15:00 h. Both 30 and 31 March were HODs. The third day, 1 April, was overcast with intermittent drizzles and solar radiation decreased abruptly to 2.6 MJ m2 as a cold front swept the region. There was no sea breeze and the sea–land temperature difference was insignificant. Consequently, the O3 concentration was rather low (less than 30–40 ppb) that day. The horizontal distribution of O3 concentration and wind field on 30 March 2000 is shown in Fig. 7. At ground level, the early morning was calm until 9:00 h. The wind speed in the entire 200 m boundary layer was below 2.5 m s1, and wind direction was northerly at both Longjin and Tsaotun from the early hours of 00:00 h to 12:00 noon. A sea breeze gathered strength before 12:00 noon to form a northwesterly wind. While wind direction at Longjin stayed northerly, wind direction at Tsaotun slowly changed to westerly with a sea breeze in the afternoon (Figs. 7 and 9). The sea breeze transported NOx, NMHC and O3 from the coastal and metropolitan areas to the southern (downwind) end of the basin (the daily maximum O3 at Chushan was 108 ppb). Higher concentrations of oxidants were usually observed near the shore in the
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Fig. 6. Surface synoptic weather map in the greater Taiwan on 30 March 2000 (courtesy Central Weather Bureau).
morning, and moved inland following the afternoon sea breeze penetration as also demonstrated by Wakamatsu et al. (1999). Above 200 m, there were clear southerly winds at both Longjin and Tsaotun. Coastal Longjin had strong wind speeds over 10.0 m s1 and Tsaotun had winds about 5.0 m s1; both increased with altitude up to 800 m. The wind calmed down to less than 1.5 m s1 and changed to northeasterly wind after sunset at 18:00 h at Longjin while dead calm continued at Tsaotun. Meanwhile, the O3 concentration started to fade away after sunset. The pattern was similar on 31 March, but of greater vertical extent. There were significant nocturnal inversion layers, as seen from Fig. 8. When the ground was heated by solar radiation during the late morning hours, with the consequent sea breeze, convective mixing occurred in the lowest part of the boundary layer. The profiles of virtual potential temperature and wind field (Figs. 8 and 9) do not provide satisfactory estimates of the mixing layer, therefore, the method proposed by Holzworth (1967) has been adopted. The mixing height in the afternoon was 600 m at Longjin and 1100 m at Tsaotun on 30 March. The height increased to 850 m at Longjin and 1600 m at Tsaotun on 31 March. The layers persisted during the afternoon hours and, as observed at the Taichung Basin, were altered in the evening by the development of a radiative inversion. It was clearly
shown that when the cold front swept the region, the frontal inversion was formed at the interface between the northerly cold air in the lower level and the southeasterly warm air, due to the influence of synoptic patterns on the upper level (also see Figs. 8 and 9). Also, this cold front with accompanying overcast and intermittent drizzles leads to the termination of episodes of ozone pollution in the region. The O3 concentrations were typically a minimum at or before dawn, rose rapidly from 07:00 h to 12:00 noon, and peaked from 12:00 noon to 16:00 h when it reached the daily maximum. The ozone concentration was rapidly depleted in the evening hours by dry deposition on the ground and by chemical reactions (Figs. 7 and 10). The diurnal profile changes of O3 were consistent with the diurnal cycle of the NO2 and NMHC. In both Longjin and Tsaotun, NO2 was high at ground level before noon on 30 and 31 March, with the concentration decreasing with height. During the daytime, especially during busy traffic hours, there was a great amount of NO and NO2 emission from traffic as well as from industry. Hence NO and NO2 concentrations increase at the ground level, and decrease with altitude. Fig. 11 shows that the concentrations of NO and NO2 are higher during the day and lower at night. This decrease in the local maximum of ozone in the late afternoon would be
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Fig. 7. Wind field and O3 concentration observed in the Taichung Basin on 30 March 2000.
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Fig. 8. Vertical profile of virtual potential temperature observed in the Taichung Basin during 30 March–1 April 2000.
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Fig. 9. Vertical profile of wind field observed in the Taichung Basin during 30 March–1 April 2000.
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Fig. 10. Vertical profile of O3 concentration observed in the Taichung Basin during 30 March–1 April, 2000.
expected, as the increased presence of NO and NO2 depleted the ozone. The higher levels of NO and NO2 at Longjin than Tsaotun are mainly because the former is
situated in an industrial area while the latter is at a rural site. At Longjin, the average NO and NO2 concentrations on the campaign days were in the range 5–42 and
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Fig. 11. (a) Vertical profiles of NO and NO2 concentrations observed at Longjin in the Taichung Basin during 30 March–1 April 2000. (b) Vertical profiles of NO and NO2 concentrations observed at Tsaotun in the Taichung Basin during 30 March–1 April 2000.
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Fig. 11 (continued ).
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Fig. 12. Vertical profile of NMHC concentration observed in the Taichung Basin during 30 March–1 April 2000.
5–25 ppb, respectively, whereas Tsaotun was in the range 0–10 and 15–36 ppb, respectively. Thus, it is conceivable that ozone is also depleted by nitrogen
oxides, predominantly by NO, O3 þNO-NO2 þO2
ð2Þ
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which is already present in the well-mixed atmosphere, before radiative cooling of the surface starts. As this reaction proceeds and NO2 concentration increases, the reaction O3 þNO2 -NO3 þO2
ð3Þ
can further deplete ozone, although its reaction rate is much slower than (2). Reaction (2) now becomes effective in depleting ozone within the lower nocturnal inversion layer (Wang, 1999; Seinfeld and Pandis, 1998). It is noted that the unusually high NO and NO2 at midlevel (400–600 m) at 15:00 h in Longjin were due to the wind blowing the pollutants from nearby stacks (250 m tall) of the Taichung power plant. The vertical profiles of NMHC during the campaign period ranged between 0.4 and 0.6 ppm at Longjin and Table 5 Meteorological variables during HODs and NHODs in synoptic patterns Type III in the month of October 2000
Max temp (1C) Cloud cover (%) Solar rad. (MJ m2) Sun (h) RH (%) 10–15 WD (s.d.) 10–15 WS (m s1)
HODs
NHODs
32.6 23.0 14.0 7.4 72.0 22.1 1.6
30.4 58.0 11.4 7.0 75.0 15.3 2.6
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between 0.2 and 0.4 ppm at Tsaotun, are well mixed and slightly higher in nighttime than daytime (Fig. 12). With regard to ozone levels, NMHC started to decrease from late morning to afternoon. Their inter-relation can be seen in reaction (1). The existing NMHC in the boundary layer could cause high ozone pollution in the metropolitan and downwind areas. It is interesting to find that the NOx had a maximum of 30–40 ppb at 400–1200 m and NMHC had a high maximum of 0.4 ppm at 100–200 m at Tsaotun on the night of 30 March. The following day, the maximum ozone concentrations were 90–110 ppb at the southern end of the basin. Since NOx and NMHC are precursors of O3, the vertical profiles of NOx, NMHC and O3 show a high correlation in this campaign. This concurs with the studies of Aneja et al. (2000), Pisano et al. (1997) and Neu et al. (1994), in which the O3 and its precursors may be stored aloft at nighttime and mixed downward to the ground the next day. Following the approach of Milford et al. (1994), no consistent association was found between the sensitivity of ozone to reductions in NMHC versus NOx emissions in this study. The ratios of NMHC/NOx in Longjin are widely scattered between 5 and 50 with R2 ¼ 0:191; and in Tsaotun between 5 and 30 with R2 ¼ 0:023: This inconsistency could be due to high air pollution as well as complicated meteorological conditions in the Taiwan region, as mentioned in a recent study by Lu and Chang (1998).
Fig. 13. Surface synoptic weather map in the greater Taiwan area on 10 October 2000.
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Fig. 14. Wind field and O3 concentration observed in the Taichung Basin on 10 October 2000.
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Fig. 15. Vertical profile of virtual potential temperature observed in the Taichung Basin during 9–11 October 2000.
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Fig. 16. Vertical profile of wind field observed in the Taichung Basin during 9–11 October 2000.
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Fig. 17. Vertical profile of O3 concentration observed in the Taichung Basin during 9–11 October 2000.
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Fig. 18. (a) Vertical profiles of NO and NO2 concentrations observed at Tachia in the Taichung Basin during 9–11 October 2000. (b) Vertical profiles of NO and NO2 concentrations observed at Tsaotun in the Taichung Basin during 9–11 October 2000.
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Fig. 18 (continued ).
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3.2. Experimental campaign II Although the statistical results of meteorological variables for October show a great difference between HODs and NHODs, the pattern is quite different from March. HODs had much lower cloud cover, only 23.0% (60.3% less than NHODs), not even half as much as the March HODs (55.3% less). This is easily explained, since Taiwan’s rainy season is during March, whereas the middle of its dry season is during October. The average wind speed 1.6 m s1 during HODs was significantly lower (38.5% lower) than NHODs, and HODs in March (23.8% less). The standard deviation of the wind direction during HODs, 22.1, was 44.4% larger than NHODs, but slightly smaller than March HODs (Table 5). Synoptic weather pattern during 9–11 October 2000 was a typical autumn Type III. A continental anticyclone moved eastwards over the East China Sea and was located northeast of Taiwan. Weak synoptic easterly winds covered the area (Fig. 13). The sky was partly cloudy with solar radiation reaching 12.6 MJ m2 on 9 October, similar to campaign I. The land temperature in Taichung City ranged from 24.01C to 31.61C. This gave maximum sea–land temperature differences of 3.51C (the sea temperature showed a 1.31C range over the day from a minimum of 26.61C at 05:00 h). The second and third days were both partly cloudy, with solar radiation 10.0 and 10.6 MJ m2, respectively. The maximum sea–land temperature differences were 3.01C and 2.51C. There were clear sea-breeze circulations in the region during the campaign period. The horizontal distribution of the wind field and O3 is shown in Fig. 14. Fig. 15 demonstrates the vertical section of winds, showing that on the first day the coastal Tachia site was under northeasterly wind in the morning while the Tsaotun Basin was much more calm. There was a sea breeze in the afternoon in the basin. The second and third days clearly show that a significant southerly land breeze dominated the region before sunrise from ground level upwards between 00:00 and 06:00 h. Subsequently, a sea breeze gathered strength by 12:00 noon, to form a significant northerly flow in the afternoon between 12:00 and 18:00 h. The wind speed in the entire boundary layer was below 3.5 m s1 with variable wind directions. The whole region was covered by an ozone concentration around 30 ppb before sunrise. At 12:00 noon, once the sea breeze developed, the concentration increased to 80 ppb in the metropolitan area of the center basin. Then, the southerly sea breeze transported O3 along with NOx and NMHC to the downwind end of the basin after 15:00 h (the readings at Chushan was 114 and 102 ppb for 9 and 10 October, respectively). By 18:00 h the ozone concentration decreased to less than 30 ppb (Figs. 14 and 16).
By the Holzworth formula, it was estimated that the mixing layers in the coastal Tachia were between 550 and 650 m, while in the basin center at Tsaotun the layers were between 650 and 900 m. The mixing layer heights were lower than during the spring Campaign I. There was a clear nocturnal inversion overnight (shown in Fig. 15). In this latter period the O3 concentration declined to 30 ppb at ground level. Fig. 17 shows that there were moderate maxima of 50–60 ppb at an altitude of 100–800 m from 10:00 until 14:00 h at the upwind Tachia site, and higher maxima of 80–90 ppb at 100– 800 m from 11:00 h until 15:00 h at the downwind Tsaotun site for all 3 days. The sea breeze components also contributed to the formation of higher ozone concentration during the afternoon in the southern end of the basin (the daily maximum O3 at Chushan were 11, 102 and 83 ppb, respectively). Fig. 18 shows that the concentrations of NO and NO2 were higher during the daytime and decreased at nighttime both at Tachia and Tsaotun. At ground level, coastal Tachia is higher than the basin site Tsaotun, which has already been discussed in the campaign I case. The NMHC profiles are not presented here due to failure to meet the standard US EPA procedures of quality assurance and control. From these two experimental campaigns, an initial conclusion can be drawn that during the Type III pattern the mixing layer is higher in spring and lower in autumn. The mixing layer is an important parameter associated with air pollutants. Normally, higher mixing layers induce better dispersion of pollutants released from surface sources (Kassomenos et al., 1995; Elsom and Chandler, 1978). Consequently, the ozone concentrations are higher in autumn than in spring. The above result could support the statistics that for 1996–2000 the frequency of occurrence HODs in Type III in autumn was higher than that in spring, as discussed earlier with reference to Table 2.
4. Conclusion It was found that the ozone concentration depends strongly on the synoptic weather pattern over the Taichung Basin. Of the 14 patterns that were examined, Type III (an anticyclone northeast of Taiwan) has the highest frequency of high ozone days of 71.7%, followed by Type VII (tropical low pressure system moving northwards toward Taiwan) at 66.7%. Furthermore, the total days of Type III (290 days) was significantly more than Type VII (207 days). Both types bring easterly winds towards Taiwan. The Taichung Basin, located on the lee side of the range, is unfavorably situated for pollutant dispersion. The afternoon sea breeze seems to contribute to the high ozone concentrations at the southern (downwind)
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end of the basin. Vertically, O3 concentrations increased with height but confined to the lowest 200–800 m in both experimental campaigns. The ozone concentrations also showed significant daily variation, with a maximum at noon and minimum at dawn. The results of this study indicate the significance of synoptic weather patterns when formulating pollution abatement strategies for sites located on the lee side of the range, such as the Taichung Basin.
Acknowledgements This research has been supported by funds from the Environmental Protection Administration (EPA) of Taiwan (EPA-89-FA11-03-231), the National Science Council of Taiwan (NSC-89-EPA-Z-029-001) and EPA of Taichung City. Special thanks are due to EPA Taiwan, the Taiwan Power Company and the Central Weather Bureau for their cooperation in providing air quality and meteorological data. The author is grateful to his colleague Dr. Gerry Rau for reading the draft of this paper. Appreciation is also extended to the author’s research assistants Miss Y.-K. Huang, Miss G. Cheng, Miss J.-L. Bai and Mr. K. Cheng for their work in helping with the field experiments, as well as their kind secretarial assistance.
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balloon in central Taiwan. Proceedings of the Australia– Taiwan Joint Symposium on Environment Modeling and Management, pp. 7–13. Wakamatsu, S., Uno, I., Ohara, T., Schere, K.L., 1999. A study of the relationship between photochemical ozone and its precursor emissions of nitrogen oxides and hydrocarbons in Tokyo and surrounding areas. Atmospheric Environment 33, 3097–3108. Wang, M., 1999. Atmospheric Chemistry, 2nd Edition. China Meteorological Press, Beijing, 467pp.
Synoptic weather patterns and their relationship to high ozone concentrations in the Taichung Basin Wan-Li Cheng Department of Environmental Science, Tunghai University, Taichung 407, Taiwan, ROC Received 1 September 2000; received in revised form 10 May 2001; accepted 26 May 2001
Abstract Frequent high ozone days (defined as daily maximum ozone concentration X80 ppb) during recent years in the Taichung Basin have caused much concern. High ozone days occur mainly during autumn and spring. Statistically, there is no clear linear relationship between a single meteorological variable and ozone concentration. In this study, data from 1996–2000 has shown that high ozone concentrations occur during two types of synoptic weather patterns. The first type is a continental cyclone emanating from mainland China, the southern part of it swept towards Taiwan by easterly winds. The second pattern is a tropical depression moving northwards toward the region, the northern part of it affecting Taiwan via easterly winds. Both types cover Taiwan with easterly winds, which are blocked by the Central Mountain Ranges (altitude of 2000–3000 m). The ranges create lee cyclogenesis to the west, which is unfavorable for pollutant dispersion and leads to serious air pollution episodes. The statistical results of the synoptic weather patterns in relation to ozone concentrations are based on the 5 yr data (1996–2000). This was obtained from a network of air-pollution monitoring sites in the study area, while the vertical data come from two 3-day tethersonde experimental campaigns conducted during March and October 2000, measuring air pressure, air temperature, relative humidity, wind speed and direction, non-methane hydrocarbons, NOx and O3. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Synoptic weather patterns; Ozone concentration; Tethersonde observation; Meteorological conditions
1. Introduction Taiwan is one of the newly industrialized economies. It has a population of 23 million in an area of 36,000 km2, with the majority of people living in the main metropolitan areas such as Taipei City (located in the Taipei Basin), Kaohsiung City (an industrial port city) and Taichung City (located in the Taichung Basin). This paper analyses the Taichung Basin, a region of 30 km 40 km, as part of the Central Taiwan AirQuality Management Program (CTAMP). West-central Taiwan has a number of low ranges (the main ones being the Tadu and Baqua Hills, both approximately 300 m elevation) running roughly north–south near the west coast. To their east is the magnificent Central Mountain Ranges (2000–3000 m). E-mail address: [email protected] (W.-L. Cheng).
The low and high ranges enclose the Taichung Basin, in which lie Taichung City and the surrounding urban towns (population around 3.5 million) (Fig. 1). Industrial and traffic emissions in this region have severely degraded the air quality causing high levels of pollutants and a reduction in visibility in recent years (EPA Taiwan, 2000; Tsai and Cheng, 1999). It is estimated that about 94,000 t yr1 of nitrogen oxides (NOx) are released into the air from industrial and traffic emissions. In addition, two thermal power plants nearby are the main sources of NOx pollutants influencing the Taichung Basin although the plants are outside the basin itself (29,000 t yr1 for the Taichung power plant and 15,000 t yr1 for the Tonhsiao plant). It is estimated that about 167,000 t of non-methane hydrocarbons (NMHC) were released in 1997 by industrial and traffic emissions. The study here extends previous work on the effects of winds on sulfur concentration (Cheng, 2001) and their
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 2 9 5 - 3
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Fig. 1. Location of monitoring stations over Taichung Basin (Tonhsiao power plant located 25 km north of Fengyuan).
effect on ozone. The sources of NOx and NMHC are mainly located in the central and northern parts of the basin, as seen in Figs. 2 and 3. Ozone (O3) is a secondary chemical pollutant, formed from NOx and volatile organic compounds (VOCs) (especially NMHC) in the presence of strong solar radiation. A general reaction mechanism for ozone at the boundary layer involves reacting NOx and NMHC with solar radiation: NOx þNMHCþhnþMðN2 ; O2 Þ-O3 þother photochemicals;
ð1Þ
where NOx comprises NO and NO2, NMHC is nonmethane hydrocarbon, hn denotes a photon, and the M returns the molecule to its original state. Most studies show that its formation is correlated with solar radiation, air temperature, relative humidity, wind speed and direction (Seinfeld and Pandis, 1998; Cheng and Bai, 1998; Poissant et al., 1996; Lalas et al., 1987). The ozone and its precursors can be transported downwind toward an area unfavorable for pollution
dispersion, thus creating the potential for a high ozone concentration in an area that has little local NOx and NMHC. Thus, the impact of ozone pollution is not limited to the location of the emission source (Gu. sten et al., 1988; Kambezidis et al., 1998; Hastie et al., 1999). A comparison of the meteorological variables on high ozone days during 1996–2000 is shown in Fig. 4. As in the Liu et al. (1994) study, there is no clear linear relationship between the meteorological components of cloud amount, solar radiation, relative humidity, sunshine duration, wind speed or maximum temperature with ozone concentration. Computer models, such as the Urban Airshed Model (UAM) and the Industrial Source Complex Dispersion Models (ISC), have been applied to the problem of ozone distribution in the west-central Taiwan region in recent years, but without much success (Chang and Wang, 1998). The Photochemical Grid model has achieved some results but is hindered by lack of sufficient local information (Chang, 2001). Patel and Kumar (1998) indicated that these models are deficient, particularly in handling airflow across complex terrain. Hence, this study investigates the relationship between
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Fig. 2. Estimated NOx emissions in west-central Taiwan in 1997.
Fig. 3. Estimated NMHC emissions in west-central Taiwan in 1997.
synoptic weather patterns and high ozone days in the Taichung Basin to identify those patterns correlated with higher ozone levels. These results could potentially be useful to EPA Taiwan in formulating pollution abatement strategies for sites specifically affected by certain synoptic weather patterns, such as the Taichung Basin.
ozone formation. In spring (March, April, May), the cold anticyclones decline, the wind speed decreases and the temperature rises. High ozone days occur during this season. Synoptic patterns in spring are an extension of the winter ones, but due to variable weather, the extent of ozone formation will fluctuate. In summer (June, July, August), Taiwan is dominated by the Pacific Anticyclone. Afternoon showers are frequent. Although intense solar radiation and low wind velocity are favorable to ozone formation, the showers will suppress high ozone levels. In autumn (September, October, November), the Pacific Anticyclone begins to retreat. The Siberian cold anticyclone begins to develop while the southern tropical depression moves occasionally northwards, exerting the most influence over Taiwan. Strong solar radiation with low humidity during autumn makes it the most favorable season for ozone formation. Several classifications of synoptic weather patterns in relation to air quality have been developed in recent years (Lee and Yu, 2000; Liu et al., 1994; Comrie, 1992, 1990). In this study, 14 synoptic weather patterns affecting the Taichung Basin have been categorized after a thorough analysis of daily surface synoptic charts issued by the Central Weather Bureau Taiwan during 1996–2000 (Cheng and Bai, 1998). Of these patterns, Types III and VII are the two patterns that produce the highest ozone concentrations
2. Synoptic patterns and their relationship to high ozone concentrations The geographical location of the study region is on the border between Eurasian continent and the Pacific Ocean, i.e. on the boundary between the Earth’s largest land and water masses. Climatically, it straddles the temperate and tropical zones. As such, it is affected in winter months by the middle latitude weather patterns and the Westerlies. In summer months, it is influenced by the tropical depressions and the Easterlies. During the winter months (December, January, February), Taiwan is predominantly influenced by anticyclones from Siberia. Accompanying the anticyclones are other synoptic patterns, such as passing cold fronts and warm areas in advance of a cold front. There is high wind speed, low temperature and the sky is cloudy with occasional rain, which is unfavorable for
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Fig. 4. Meteorological variables versus peak O3 (ppb) for Taichung Basin during 1996–2000.
in the Taichung Basin. Type III, where the anticyclone is located northeast of Taiwan, has weak to moderate easterly winds covering the region. Type VII, where the northern extent of the tropical depression reaches the vicinity of Taiwan, also has weak to moderate easterly winds.
During easterly winds, such as those caused by Types III and VII, the winds are blocked by the Central Mountain Ranges and split into two currents which pass around the ranges to the north and south, while the depleting air to the west creates the characteristic lee trough (Lin, 2001). This could also modify
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micro-synoptic features significantly through such processes as lee cyclogenesis, which is unfavorable for pollutant dispersion, resulting in high ozone concentrations (Lee and Yu, 2000). Table 1 illustrates the frequency of high ozone occurrences. Among the 14 patterns, Type III has the highest frequency at 71.7% (208 out of 290 days), followed by Type VII at 66.7% (138 out of 207 days). It is also found that among Type III, high ozone pollution days (HODs) mainly occurred in autumn with the highest frequency (86.8%), followed by spring (71.1%), as shown in Table 2. A HOD is defined here as a day with a maximum ozone concentration greater than 80 ppb. The remaining days are non-HODs (NHODs). The concentration level of 80 ppb is the pollution critical-threshold index widely used by many countries such as Brazil, Canada, China, Japan, Poland, etc., but not the USA, which uses a level of 120 ppb. Statistical analysis of synoptic pattern Type III in the period 1996–2000 shows that there are significant differences between HODs and NHODs in meteorological variables (Table 3). HODs occur under light wind speeds, averaging 2.0 m s1 (23.0% less than NHODs), and show a large standard deviation in wind direction of 23.5 (30.6% more than NHODs). The maximum temperature during HODs was 28.81C (7.9% higher than NHODs), and the average cloud cover was low 38.2% (18.2% less than NHODs).
3. Tethersonde experimental campaigns In order to monitor high ozone concentration patterns for Type III as mentioned above, two experi-
Table 1 The frequency of occurrence of high ozone days with different synoptic weather patterns in the period 1996–2000
Type II Type III Type VII
Days
HODs
Frequency (%)
249 290 207
92 208 138
36.9 71.7 66.7
mental campaigns were conducted for three consecutive days: 30 March–1 April 2000 with a typical spring Type III pattern, and 9–11 October 2000 with a typical autumn Type III pattern. The meteorological conditions over the study area for these 3-day periods were analyzed, and the vertical profiles of nitrogen oxide and ozone were examined, including their diurnal variation. The tethersonde has a balloon-borne radiosonde facility to observe air pressure, air temperature, relative humidity, wind speed and direction. The radiosonde was supported by a 3 m3 hydrogen balloon, tethered on a Kevlar line and controlled by an electric capstan. On the ground were additional air pollutant (O3, NOx, NMHC) analyzers. The sampling altitudes were at ground level, 50, 100, 250, 400, and 600 m for the coastal sites at Longjin and Tachia, and ground level, 100, 300, 500, and 1200 m for the basin center site at Tsaotun. Samples were taken every 3 h, with one and a half hours required to complete the observation and analysis for one round. An operational diagram of the experimental equipment is shown in Fig. 5. The ozone analyzer used was the Advanced Pollution Instrumentation, Inc. Model API 400. The instrument is designed with a built-in daily calibration system with a two-point (0, 400ppb) calibration, and a multipoint (0, 100, 200, 300, 400 ppb) calibration prior to use, according to the manufacturer’s calibration procedure. The accuracy of this instrument is 71 ppb and the time required for the analysis of one sample was approximately 5 min. The nitrogen oxides analyzer used was the Advanced Pollution Instrumentation, Inc. API Model 200A. The Model 200A utilizes one small diameter photomultiplier tube and one reaction chamber which are time-multiplexed for NO and NOx measurements. The difference between the NOx and NO allows the calculation of NO2. The instrument was also calibrated prior to use, according to the manufacturer’s procedure. Its accuracy is 70.5% of the reading and the required time for the analysis was also about 5 min. The methane/non-methane hydrocarbon analyzer is made by the Dasibi Environmental Corporation, Model Table 3 Meteorological variables during HODs and NHODs in synoptic patterns Type III in the period 1996–2000
Table 2 The frequency of occurrence of high ozone days with synoptic pattern Type III in the period 1996–2000
Winter Spring Summer Autumn
Days
HODs
Frequency (%)
85 81 F 91
48 114 F 79
56.5 71.1 F 86.8
Max temp (1C) Cloud cover (%) Solar rad. (MJ m2) Sun (h) RH (%) 10–15 WD (s.d.) 10–15 WS (m s1)
HODs
NHODs
28.8 38.2 12.1 7.1 73.2 23.5 2.0
26.7 46.7 11.6 6.9 75.2 18.0 2.6
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W.-L. Cheng / Atmospheric Environment 35 (2001) 4971–4994 Table 4 Meteorological variables during HODs and NHODs in synoptic patterns Type III in the month of March 2000
Max temp (1C) Cloud cover (%) Solar rad. (MJ m2) Sun (h) RH (%) 10–15 WD (s.d.) 10–15 WS (m s1)
Fig. 5. Operational diagram of instruments.
302. The analyzer was calibrated prior to use, checked after each start up and also periodically as the instrument follows an automatic monitoring schedule. Its accuracy is 1.0% of full scale and the required time for the analysis was also about 5 min. All data used in this paper have undergone standard US EPA procedures of quality assurance and control to minimize errors, such as sensitivity analysis, computerized checks, reality checks, sample calculation, statistical checks, independent audits, etc. The instrumentation and the methodologies used to monitor the CTAMP tethersonde data sets have been described elsewhere (Cheng, 2000; Tsuang et al., 2000). 3.1. Experimental campaign I Statistical analysis of the synoptic pattern Type III in March 2000 also shows significant differences in meteorological variables between HODs and NHODs (Table 4). Although the wind speed was identical (2.1 m s1), HODs had a larger standard deviation in wind direction of 28.0 (56.4% more than NHODs). HODs’ maximum temperatures were 28.11C (15.2% higher than NHODs), and had a lower average cloud cover of 51.5% (20.2% less than NHODs). During 30 and 31 March 2000, a continental anticyclone moving eastwards over the East China Sea had its center located northeast of Taiwan, with weak
HODs
NHODs
28.1 51.5 12.3 7.3 61.5 28.0 2.1
24.4 64.5 10.4 5.5 71.5 17.9 2.1
easterly winds covering the area. This was a typical spring Type III pattern, with relatively weak pressure center (Fig. 6). 30 March 2000 was a clear day with solar radiation reaching 13.2 MJ m2. The temperature at Taichung City ranged from 18.11C to 27.81C. This gave maximum sea–land temperature differences of 5.41C (the sea temperature showed a 4.91C range over the day from a minimum of 17.01C at 06:00 h). All times hereafter are referenced to Taiwan Central Standard Time, which is 8 h ahead of UT. The second day was partly cloudy, with solar radiation slightly reduced to 11.3 MJ m2, and the maximum sea–land temperature difference was 4.71C at 15:00 h. In these two days, clear sea breeze circulations developed as shown from the wind field in the region (Fig. 7) and at the vertical profile (Fig. 8). On the afternoon of 30 and 31 March, the sea breeze inflow reached altitudes of 200–400 m. At ground level, the O3 concentration became highest at the southern (downwind) end of the basin at 15:00 h. Both 30 and 31 March were HODs. The third day, 1 April, was overcast with intermittent drizzles and solar radiation decreased abruptly to 2.6 MJ m2 as a cold front swept the region. There was no sea breeze and the sea–land temperature difference was insignificant. Consequently, the O3 concentration was rather low (less than 30–40 ppb) that day. The horizontal distribution of O3 concentration and wind field on 30 March 2000 is shown in Fig. 7. At ground level, the early morning was calm until 9:00 h. The wind speed in the entire 200 m boundary layer was below 2.5 m s1, and wind direction was northerly at both Longjin and Tsaotun from the early hours of 00:00 h to 12:00 noon. A sea breeze gathered strength before 12:00 noon to form a northwesterly wind. While wind direction at Longjin stayed northerly, wind direction at Tsaotun slowly changed to westerly with a sea breeze in the afternoon (Figs. 7 and 9). The sea breeze transported NOx, NMHC and O3 from the coastal and metropolitan areas to the southern (downwind) end of the basin (the daily maximum O3 at Chushan was 108 ppb). Higher concentrations of oxidants were usually observed near the shore in the
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Fig. 6. Surface synoptic weather map in the greater Taiwan on 30 March 2000 (courtesy Central Weather Bureau).
morning, and moved inland following the afternoon sea breeze penetration as also demonstrated by Wakamatsu et al. (1999). Above 200 m, there were clear southerly winds at both Longjin and Tsaotun. Coastal Longjin had strong wind speeds over 10.0 m s1 and Tsaotun had winds about 5.0 m s1; both increased with altitude up to 800 m. The wind calmed down to less than 1.5 m s1 and changed to northeasterly wind after sunset at 18:00 h at Longjin while dead calm continued at Tsaotun. Meanwhile, the O3 concentration started to fade away after sunset. The pattern was similar on 31 March, but of greater vertical extent. There were significant nocturnal inversion layers, as seen from Fig. 8. When the ground was heated by solar radiation during the late morning hours, with the consequent sea breeze, convective mixing occurred in the lowest part of the boundary layer. The profiles of virtual potential temperature and wind field (Figs. 8 and 9) do not provide satisfactory estimates of the mixing layer, therefore, the method proposed by Holzworth (1967) has been adopted. The mixing height in the afternoon was 600 m at Longjin and 1100 m at Tsaotun on 30 March. The height increased to 850 m at Longjin and 1600 m at Tsaotun on 31 March. The layers persisted during the afternoon hours and, as observed at the Taichung Basin, were altered in the evening by the development of a radiative inversion. It was clearly
shown that when the cold front swept the region, the frontal inversion was formed at the interface between the northerly cold air in the lower level and the southeasterly warm air, due to the influence of synoptic patterns on the upper level (also see Figs. 8 and 9). Also, this cold front with accompanying overcast and intermittent drizzles leads to the termination of episodes of ozone pollution in the region. The O3 concentrations were typically a minimum at or before dawn, rose rapidly from 07:00 h to 12:00 noon, and peaked from 12:00 noon to 16:00 h when it reached the daily maximum. The ozone concentration was rapidly depleted in the evening hours by dry deposition on the ground and by chemical reactions (Figs. 7 and 10). The diurnal profile changes of O3 were consistent with the diurnal cycle of the NO2 and NMHC. In both Longjin and Tsaotun, NO2 was high at ground level before noon on 30 and 31 March, with the concentration decreasing with height. During the daytime, especially during busy traffic hours, there was a great amount of NO and NO2 emission from traffic as well as from industry. Hence NO and NO2 concentrations increase at the ground level, and decrease with altitude. Fig. 11 shows that the concentrations of NO and NO2 are higher during the day and lower at night. This decrease in the local maximum of ozone in the late afternoon would be
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Fig. 7. Wind field and O3 concentration observed in the Taichung Basin on 30 March 2000.
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Fig. 8. Vertical profile of virtual potential temperature observed in the Taichung Basin during 30 March–1 April 2000.
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Fig. 9. Vertical profile of wind field observed in the Taichung Basin during 30 March–1 April 2000.
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Fig. 10. Vertical profile of O3 concentration observed in the Taichung Basin during 30 March–1 April, 2000.
expected, as the increased presence of NO and NO2 depleted the ozone. The higher levels of NO and NO2 at Longjin than Tsaotun are mainly because the former is
situated in an industrial area while the latter is at a rural site. At Longjin, the average NO and NO2 concentrations on the campaign days were in the range 5–42 and
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Fig. 11. (a) Vertical profiles of NO and NO2 concentrations observed at Longjin in the Taichung Basin during 30 March–1 April 2000. (b) Vertical profiles of NO and NO2 concentrations observed at Tsaotun in the Taichung Basin during 30 March–1 April 2000.
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Fig. 11 (continued ).
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Fig. 12. Vertical profile of NMHC concentration observed in the Taichung Basin during 30 March–1 April 2000.
5–25 ppb, respectively, whereas Tsaotun was in the range 0–10 and 15–36 ppb, respectively. Thus, it is conceivable that ozone is also depleted by nitrogen
oxides, predominantly by NO, O3 þNO-NO2 þO2
ð2Þ
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which is already present in the well-mixed atmosphere, before radiative cooling of the surface starts. As this reaction proceeds and NO2 concentration increases, the reaction O3 þNO2 -NO3 þO2
ð3Þ
can further deplete ozone, although its reaction rate is much slower than (2). Reaction (2) now becomes effective in depleting ozone within the lower nocturnal inversion layer (Wang, 1999; Seinfeld and Pandis, 1998). It is noted that the unusually high NO and NO2 at midlevel (400–600 m) at 15:00 h in Longjin were due to the wind blowing the pollutants from nearby stacks (250 m tall) of the Taichung power plant. The vertical profiles of NMHC during the campaign period ranged between 0.4 and 0.6 ppm at Longjin and Table 5 Meteorological variables during HODs and NHODs in synoptic patterns Type III in the month of October 2000
Max temp (1C) Cloud cover (%) Solar rad. (MJ m2) Sun (h) RH (%) 10–15 WD (s.d.) 10–15 WS (m s1)
HODs
NHODs
32.6 23.0 14.0 7.4 72.0 22.1 1.6
30.4 58.0 11.4 7.0 75.0 15.3 2.6
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between 0.2 and 0.4 ppm at Tsaotun, are well mixed and slightly higher in nighttime than daytime (Fig. 12). With regard to ozone levels, NMHC started to decrease from late morning to afternoon. Their inter-relation can be seen in reaction (1). The existing NMHC in the boundary layer could cause high ozone pollution in the metropolitan and downwind areas. It is interesting to find that the NOx had a maximum of 30–40 ppb at 400–1200 m and NMHC had a high maximum of 0.4 ppm at 100–200 m at Tsaotun on the night of 30 March. The following day, the maximum ozone concentrations were 90–110 ppb at the southern end of the basin. Since NOx and NMHC are precursors of O3, the vertical profiles of NOx, NMHC and O3 show a high correlation in this campaign. This concurs with the studies of Aneja et al. (2000), Pisano et al. (1997) and Neu et al. (1994), in which the O3 and its precursors may be stored aloft at nighttime and mixed downward to the ground the next day. Following the approach of Milford et al. (1994), no consistent association was found between the sensitivity of ozone to reductions in NMHC versus NOx emissions in this study. The ratios of NMHC/NOx in Longjin are widely scattered between 5 and 50 with R2 ¼ 0:191; and in Tsaotun between 5 and 30 with R2 ¼ 0:023: This inconsistency could be due to high air pollution as well as complicated meteorological conditions in the Taiwan region, as mentioned in a recent study by Lu and Chang (1998).
Fig. 13. Surface synoptic weather map in the greater Taiwan area on 10 October 2000.
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Fig. 14. Wind field and O3 concentration observed in the Taichung Basin on 10 October 2000.
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Fig. 15. Vertical profile of virtual potential temperature observed in the Taichung Basin during 9–11 October 2000.
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Fig. 16. Vertical profile of wind field observed in the Taichung Basin during 9–11 October 2000.
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Fig. 17. Vertical profile of O3 concentration observed in the Taichung Basin during 9–11 October 2000.
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Fig. 18. (a) Vertical profiles of NO and NO2 concentrations observed at Tachia in the Taichung Basin during 9–11 October 2000. (b) Vertical profiles of NO and NO2 concentrations observed at Tsaotun in the Taichung Basin during 9–11 October 2000.
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Fig. 18 (continued ).
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3.2. Experimental campaign II Although the statistical results of meteorological variables for October show a great difference between HODs and NHODs, the pattern is quite different from March. HODs had much lower cloud cover, only 23.0% (60.3% less than NHODs), not even half as much as the March HODs (55.3% less). This is easily explained, since Taiwan’s rainy season is during March, whereas the middle of its dry season is during October. The average wind speed 1.6 m s1 during HODs was significantly lower (38.5% lower) than NHODs, and HODs in March (23.8% less). The standard deviation of the wind direction during HODs, 22.1, was 44.4% larger than NHODs, but slightly smaller than March HODs (Table 5). Synoptic weather pattern during 9–11 October 2000 was a typical autumn Type III. A continental anticyclone moved eastwards over the East China Sea and was located northeast of Taiwan. Weak synoptic easterly winds covered the area (Fig. 13). The sky was partly cloudy with solar radiation reaching 12.6 MJ m2 on 9 October, similar to campaign I. The land temperature in Taichung City ranged from 24.01C to 31.61C. This gave maximum sea–land temperature differences of 3.51C (the sea temperature showed a 1.31C range over the day from a minimum of 26.61C at 05:00 h). The second and third days were both partly cloudy, with solar radiation 10.0 and 10.6 MJ m2, respectively. The maximum sea–land temperature differences were 3.01C and 2.51C. There were clear sea-breeze circulations in the region during the campaign period. The horizontal distribution of the wind field and O3 is shown in Fig. 14. Fig. 15 demonstrates the vertical section of winds, showing that on the first day the coastal Tachia site was under northeasterly wind in the morning while the Tsaotun Basin was much more calm. There was a sea breeze in the afternoon in the basin. The second and third days clearly show that a significant southerly land breeze dominated the region before sunrise from ground level upwards between 00:00 and 06:00 h. Subsequently, a sea breeze gathered strength by 12:00 noon, to form a significant northerly flow in the afternoon between 12:00 and 18:00 h. The wind speed in the entire boundary layer was below 3.5 m s1 with variable wind directions. The whole region was covered by an ozone concentration around 30 ppb before sunrise. At 12:00 noon, once the sea breeze developed, the concentration increased to 80 ppb in the metropolitan area of the center basin. Then, the southerly sea breeze transported O3 along with NOx and NMHC to the downwind end of the basin after 15:00 h (the readings at Chushan was 114 and 102 ppb for 9 and 10 October, respectively). By 18:00 h the ozone concentration decreased to less than 30 ppb (Figs. 14 and 16).
By the Holzworth formula, it was estimated that the mixing layers in the coastal Tachia were between 550 and 650 m, while in the basin center at Tsaotun the layers were between 650 and 900 m. The mixing layer heights were lower than during the spring Campaign I. There was a clear nocturnal inversion overnight (shown in Fig. 15). In this latter period the O3 concentration declined to 30 ppb at ground level. Fig. 17 shows that there were moderate maxima of 50–60 ppb at an altitude of 100–800 m from 10:00 until 14:00 h at the upwind Tachia site, and higher maxima of 80–90 ppb at 100– 800 m from 11:00 h until 15:00 h at the downwind Tsaotun site for all 3 days. The sea breeze components also contributed to the formation of higher ozone concentration during the afternoon in the southern end of the basin (the daily maximum O3 at Chushan were 11, 102 and 83 ppb, respectively). Fig. 18 shows that the concentrations of NO and NO2 were higher during the daytime and decreased at nighttime both at Tachia and Tsaotun. At ground level, coastal Tachia is higher than the basin site Tsaotun, which has already been discussed in the campaign I case. The NMHC profiles are not presented here due to failure to meet the standard US EPA procedures of quality assurance and control. From these two experimental campaigns, an initial conclusion can be drawn that during the Type III pattern the mixing layer is higher in spring and lower in autumn. The mixing layer is an important parameter associated with air pollutants. Normally, higher mixing layers induce better dispersion of pollutants released from surface sources (Kassomenos et al., 1995; Elsom and Chandler, 1978). Consequently, the ozone concentrations are higher in autumn than in spring. The above result could support the statistics that for 1996–2000 the frequency of occurrence HODs in Type III in autumn was higher than that in spring, as discussed earlier with reference to Table 2.
4. Conclusion It was found that the ozone concentration depends strongly on the synoptic weather pattern over the Taichung Basin. Of the 14 patterns that were examined, Type III (an anticyclone northeast of Taiwan) has the highest frequency of high ozone days of 71.7%, followed by Type VII (tropical low pressure system moving northwards toward Taiwan) at 66.7%. Furthermore, the total days of Type III (290 days) was significantly more than Type VII (207 days). Both types bring easterly winds towards Taiwan. The Taichung Basin, located on the lee side of the range, is unfavorably situated for pollutant dispersion. The afternoon sea breeze seems to contribute to the high ozone concentrations at the southern (downwind)
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end of the basin. Vertically, O3 concentrations increased with height but confined to the lowest 200–800 m in both experimental campaigns. The ozone concentrations also showed significant daily variation, with a maximum at noon and minimum at dawn. The results of this study indicate the significance of synoptic weather patterns when formulating pollution abatement strategies for sites located on the lee side of the range, such as the Taichung Basin.
Acknowledgements This research has been supported by funds from the Environmental Protection Administration (EPA) of Taiwan (EPA-89-FA11-03-231), the National Science Council of Taiwan (NSC-89-EPA-Z-029-001) and EPA of Taichung City. Special thanks are due to EPA Taiwan, the Taiwan Power Company and the Central Weather Bureau for their cooperation in providing air quality and meteorological data. The author is grateful to his colleague Dr. Gerry Rau for reading the draft of this paper. Appreciation is also extended to the author’s research assistants Miss Y.-K. Huang, Miss G. Cheng, Miss J.-L. Bai and Mr. K. Cheng for their work in helping with the field experiments, as well as their kind secretarial assistance.
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